Chromatic dispersion effect in optical fiber will extend width of the transmission pulse, and then cause bit error. In order to eliminate this influence, a chromatic dispersion compensation module shall be added. With respect to a system having time varying chromatic dispersion, real-time monitoring and compensation for the chromatic dispersion are required. The literature OFC2008 OThW2 “Optical Performance Monitoring from FIR Filter Coefficients in Coherent Receivers” provides a real-time chromatic dispersion monitoring method based on filter coefficient analysis. FIG. 1 illustrates a chromatic dispersion monitor for the real-time chromatic dispersion monitoring method. As shown in FIG. 1, the chromatic dispersion monitoring device of the prior art comprises an adaptive filter 101, a chromatic dispersion correlation amount calculating unit 102 and a chromatic dispersion monitor 105, wherein the chromatic dispersion monitor 105 comprises a phase extraction unwrapping unit 103 and a chromatic dispersion estimating unit 104.
The adaptive filter 101 can be a single FIR adaptive filter (single-polarization system), or a set of adaptive filters having butterfly structure (double-polarization system). The chromatic dispersion correlation amount calculating unit 102 extracts chromatic dispersion correlation component H(ω) with certain algorithm based on the filter coefficient. H(ω) is a set of discrete sequences represented as h1, h2, . . . , hN with the length equal to the filter coefficient length N. In the sequence h1, h2, . . . , hN, respective element has the form of Aej(ω2a+ωb+c)+n, wherein α is in a one-to-one correspondence to the chromatic dispersion amount in the system, A, b and c are constants unrelated to the chromatic dispersion, and n is a noise item. The phase extraction unwrapping unit 103 extracts a phase of H(ω); as the extracted phase is always within [0, 2π), and the actual value of quadratic curve may exceed the range, the phase extraction unwrapping unit 103 is required for unwrapping at the same time. The chromatic dispersion estimating unit 104 obtains the above parameter α through quadratic fitting, and hence estimates the chromatic dispersion. This method balances the system with the adaptive filter, extracts chromatic dispersion correlated portion based on the converged filter coefficient, and estimates the parameter α through quadratic fitting of phase angle, so as to obtain the chromatic dispersion amount in the system. This estimation method has the problem of phase wrap, i.e., once the phase wrap is wrong, the chromatic dispersion amount will not be estimated correctly.